A quantum Langevin model, like models from optomechanics, was developed for phonons. It predicts intermodulation phonon sidebands (IPSs) in anharmonic crystals. Ab initio calculations of anharmonic phonons in rock-salt NaBr showed these spectral features as many-body effects. Modern inelastic neutron scattering measurements on a crystal of NaBr at 300 K revealed diffuse intensity at high phonon energy from a predicted upper IPS. The transverse optical (TO) part of the new features originates from phonon intermodulation between the transverse acoustic (TA) and TO phonons. The longitudinal optical spectral features originate from three-phonon coupling between the TA modes and the TO lattice modes. The partner lower IPS proves to be an intrinsic localized mode. Interactions with the thermal bath broaden and redistribute the spectral weight of the IPS pair. These sidebands are a probe of the anharmonicity and quantum noise of phonons in NaBr and suggest novel interactions between photons and phonons.

We utilize simulations of electron scattering by a chain of dynamical quantum spins to analyze the interplay between the spin transfer effect and the magnetization dynamics. We show that the complex interactions between the spin-polarized electrons and the dynamical states of the local spins can be decomposed into separate processes involving electron reflection and transmission, as well as absorption and emission of magnons—the quanta of magnetization dynamics. Analysis shows that these processes are substantially constrained by the energy and momentum conservation laws, resulting in a significant dependence of spin transfer on the electron's energy and the dynamical state of the local spins. Our results suggest that exquisite control of spin transfer efficiency and of the resulting dynamical magnetization states may be achievable by tailoring the spectral characteristics of the conduction electrons and of the magnetic systems.

We discuss theoretically static and dynamical properties of XY and Heisenberg antiferromagnets on the triangular lattice with random vacancies and substitutional spins. It is shown that the distortion of ${120}^{\circ }$ magnetic order produced by a single defect is described by electrostatic equations for a field of an electrically neutral complex of six charges located around the impurity. The first finite term in the multipole expansion of this field is the octupole moment which decays as $1/r^3$ with the distance $r$. The linearity of equations allows to describe analytically the distortion of the long-range magnetic order at a small concentration $c$ of defects. We obtain analytically renormalization of the elastic neutron scattering cross section and the magnon spectrum ${\epsilon }_{\mathbf{k}}$ in the leading order in $c$. We find that the scattering on impurities renormalizes weakly the bare spectrum ${\epsilon }_{\mathbf{k}}\propto k$ at $k\gg \sqrt{c}$. However the renormalization is substantial of the long-wavelength magnon spectrum at $k\ll \sqrt{c}$: ${\epsilon }_{\mathbf{k}}\propto \sqrt{c/ln(1/k)}$ at $k\to 0$ and there is a parametrically large region in which magnons with not too small momenta are overdamped and localized. This strong modification of the long-wavelength spectrum leads to the stabilization of the slightly distorted magnetic long-range order at $T<T_N\sim S^{2}J/ln(1/c)$ and to the considerable change in the density of states and in the specific heat. The overdamped modes arise also in quasi-2D spin systems on a stacked triangular lattice.

We theoretically investigate the vortex spin Hall effect, i.e., a novel spin Hall effect driven by the motion of superconducting vortices, by focusing on the role of superconducting fluctuations. Within the BCS-Gor'kov microscopic approach combined with the Kubo formula, we find a strong similarity between the vortex spin Hall effect and the vortex Nernst/Ettingshausen effect. Calculated temperature dependence of the voltage signal due to the inverse vortex spin Hall effect exhibits a strong enhancement by vortex fluctuations. This result not only provides a possible explanation for a prominent peak found in the spin Seebeck effect in a NbN/${\mathrm{Y}}_3{\mathrm{Fe}}_5{\mathrm{O}}_{12}$ system but also leads to a proposal of new experiments using other superconductors with strong fluctuations, such as cuprate or iron-based superconductors.

The thermodynamic properties of hexagonal-close-packed iron $\left(\epsilon \text{−}\mathrm{Fe}\right)$ are essential for investigating the internal structure and dynamic properties of planetary cores. Despite their importance to planetary sciences, experimental investigations of $\epsilon \text{−}\mathrm{Fe}$ at relevant conditions are still challenging. Therefore, ab initio calculations are crucial to elucidating the thermodynamic properties of this system. Here, we use a free energy calculation scheme based on the phonon gas model compatible with temperature-dependent phonon frequencies. We investigate the effects of electronic thermal excitations, which introduces a temperature dependence on phonon frequencies, and the implication for the thermodynamic properties of $\epsilon \text{−}\mathrm{Fe}$ at extreme pressure $\left(P\right)$ and temperature $\left(T\right)$ conditions. We disregard phonon-phonon interactions, i.e., anharmonicity and their effect on phonon frequencies. Nevertheless, the current scheme is also applicable to $T$-dependent anharmonic frequencies. We conclude that the impact of thermal electronic excitations on vibrational properties is not significant up to ∼4000 K at 200 GPa but should not be ignored at higher temperatures or pressures. However, the static free energy $F_{\mathrm{st}}$ must always include the effect of thermal excitation fully in a continuum of $T$. Our results for isentropic equations of state show good agreement with data from recent ramp compression experiments up to 1400 GPa conducted at the National Ignition Facility.

Lattice thermal conductivity (${\kappa }_{\mathrm{lat}}$) of $\mathrm{Mg}\mathrm{Si}{\mathrm{O}}_3$ postperovskite (MgPPv) under the Earth's lower mantle high pressure-temperature conditions is studied using the phonon quasiparticle approach by combing ab initio molecular dynamics and lattice dynamics simulations. Phonon lifetimes are extracted from the phonon quasiparticle calculations, and the phonon group velocities are computed from the anharmonic phonon dispersions, which, in principle, capture full anharmonicity. It is found that throughout the lowermost mantle, including the D” region, ${\kappa }_{\mathrm{lat}}$ of MgPPv is ∼25% larger than that of $\mathrm{Mg}\mathrm{Si}{\mathrm{O}}_3$ perovskite (MgPv), mainly due to MgPPv's higher phonon velocities. Such a difference in phonon velocities between the two phases originates in the MgPPv's relatively smaller primitive cell. Systematic results of temperature and pressure dependences of both MgPPv's and MgPv's ${\kappa }_{\mathrm{lat}}$ are demonstrated.

We must protect inherently fragile quantum data to unlock the potential of quantum technologies. A pertinent concern in schemes for quantum storage is their potential for near-term implementation. Since Heisenberg ferromagnets are readily available, we investigate their potential for robust quantum storage. We propose to use permutation-invariant quantum codes to store quantum data in Heisenberg ferromagnets, because the ground space of any Heisenberg ferromagnet must be symmetric under any permutation of the underlying qubits. By exploiting an area law on the expected energy of Pauli errors, we show that increasing the effective dimension of Heisenberg ferromagnets can improve the storage lifetime. When the effective dimension of Heisenberg ferromagnets is maximal, we also obtain an upper bound for the storage error. This result relies on perturbation theory, where we use Davis' divided difference representation for Fréchet derivatives along with the recursive structure of these divided differences. Our numerical bounds allow us to better understand how quantum memory lifetimes can be enhanced in Heisenberg ferromagnets.

Rare-earth chromates have always been of interest due to temperature-induced magnetization reversal and spin-reorientation phase transitions (SRPTs). In orthochromates containing magnetic rare earths, the spin configuration is supposed to undergo a characteristic changeover across the SRPT followed by an independent ordering of rare-earth moments leading to polar order. However, due to the presence of nearly 14% of highly neutron-absorbing isotope ${}^{149}\mathrm{Sm}$ in natural Sm based compounds, correct magnetic structure determination of ${\mathrm{SmCrO}}_3$ through neutron diffraction measurements has been a challenge. In the present study we investigate the pre- and post-SRPT spin configurations in well characterized ${\mathrm{SmCrO}}_3$ through time of flight neutron diffraction measurements carried out in zero field at the high-resolution high-flux WISH beam line of ISIS, in the United Kingdom. Magnetization measurement shows a canted antiferromagnetic phase transition at $T_{N1}=192\mathrm{K}$, giving rise to a weak ferromagnetism, which undergoes a SRPT at 37 K. Rietveld analysis of the neutron powder diffraction data shows that below $T_{N1}=192\mathrm{K}$ the ${\mathrm{Cr}}^{3+}$ and ${\mathrm{Sm}}^{3+}$ moments order in a $P{b}^{\prime }{n}^{\prime }m$:${\mathrm{\Gamma }}_4$(${\mathit{G}}_{\mathit{x}},A_y,F_Z;F_Z^R$) spin configuration with their tiny ferromagnetic components $F_Z$ and $F_Z^R,$ giving rise to weak ferromagnetism. Below 37 K the $P{b}^{\prime }{n}^{\prime }m$:${\mathrm{\Gamma }}_4$(${\mathit{G}}_{\mathit{x}},A_y,F_Z;F_Z^R$) configuration transforms to $Pb{n}^{\prime }{m}^{\prime }:{\mathrm{\Gamma }}_2$($F_x,C_y,{\mathit{G}}_{\mathit{Z}};F_x^R,{\mathit{C}}_{\mathit{y}}^{\mathit{R}}$) as a result of continuous rotation of ${\mathrm{Cr}}^{3+}$ moments, while approaching SRPT below $T_{N1}$. At still lower temperatures the $Pb{n}^{\prime }{m}^{\prime }:{\mathrm{\Gamma }}_2$($F_x,C_y,{\mathit{G}}_{\mathit{Z}};F_x^R,{\mathit{C}}_{\mathit{y}}^{\mathit{R}}$) phase transforms to polar phases, either the $P{2}_1^{\prime }{2}_1^{\prime }{2}_1:{\mathrm{\Gamma }}_{26}$($C_x,{\mathit{G}}_{\mathit{y}},F_z;{\mathit{C}}_{\mathit{x}}^{\mathit{R}},A_y^R,F_z^R$) or the $P{n}^{\prime }a{2}_1^{\prime }:{\mathrm{\Gamma }}_{27}$($F_x,C_y,{\mathit{G}}_{\mathit{z}};F_x^R,{\mathit{C}}_{\mathit{y}}^{\mathit{R}},G_z^R$) phase, as a result of independent antiferromagnetic ordering of ${\mathrm{Sm}}^{3+}$ moments at $T_{N2}<4\mathrm{K}$ through ${\mathrm{Sm}}^{3+}\text{−}{\mathrm{Sm}}^{3+}$ direct interaction. Our result of the transformation of ${\mathrm{SmCrO}}_3$ from ${\mathrm{\Gamma }}_4$ to ${\mathrm{\Gamma }}_2$ below SRPT is in contradiction with the ${\mathrm{\Gamma }}_1$($A_x,{\mathit{G}}_{\mathit{y}},C_Z;C_z^R$) spin configuration as reported in Tripathi et al. [Phys. Rev. B 96, 174421 (2017)]. This issue has been independently settled through ground-state energy calculation using spin-dependent density functional theory confirming the ${\mathrm{\Gamma }}_2$ spin configuration to be of lower energy as compared to that of the ${\mathrm{\Gamma }}_1$. The role of magnetocrystalline anisotropy in the occurrence of SRPT has been discussed.

We introduce a novel mean-field theory (MFT) around the exactly soluble two-leg ladder limit for the planar quantum compass model (QCM). In contrast to usual MFT, our construction respects the stringent constraints imposed by emergent, lower (here $d=1$) dimensional symmetries of the QCM. Specializing our construction to the QCM on a periodic four-leg ladder, we find that a first-order transition separates two mutually dual Ising nematic phases, in good accord with state-of-the-art numerics for the planar QCM. One pseudo-spin-flip excitation in the ordered phase turns out to be two (Jordan-Wigner) fermion bound states, reminiscent of spin waves in spin-$1/2$ Heisenberg chains. We discuss the novel implications of our work on (1) the emergence of coupled orbital and magnetic ordered and liquidlike disordered phases and (2) a rare instance of orbital-spin separation in $d>1$, in the context of a Kugel-Khomskii view of multiorbital Mott insulators.

Elastic strain is known to spatially modulate the wave-function overlap of the atoms on the lattice and can drastically alter the properties of the quasiparticles. For example, strain in Dirac matter can be interpreted as an elastic gauge field inducing Landau levels. We here propose a general method resolving the dispersion of the strain-induced Landau levels in two-dimensional Dirac materials, regardless of the particular space dependence of the applied strain. We illustrate such a method with the twist-induced magnon Landau levels in honeycomb quantum magnet nanoribbons. For ferromagnetic nanoribbons, dispersive Dirac-Landau levels are induced in the center of the magnon bands, while for antiferromagnetic nanoribbons, the twist results in dispersive equidistant Landau levels at the top of the magnon bands.

Ferromagnetic superconductor URhGe has orthorhombic structure and possesses spontaneous magnetization along the $c$ axis. Magnetic field directed along the $b$ axis suppresses ferromagnetism in the $c$ direction and leads to a metamagnetic transition into polarized paramagnetic state in the $b$ direction. The theory of these phenomena based on the specific magnetic anisotropy of this material in the $(b,c)$ plane is given. Line of the first order metamagnetic transition ends at a critical point. The Van der Waals-type description of behavior of physical properties near this point is developed. The triplet superconducting state destroyed by orbital effect is recreated in the vicinity of the transition. It is shown that the reentrance of superconductivity is caused by the sharp increase of magnetic susceptibility in the $b$ direction near the metamagnetic transition. The specific behavior of the upper critical field in the direction of spontaneous magnetization in UCoGe and in $\mathrm{U}{\mathrm{Ge}}_2$ related to the field dependence of magnetic susceptibility is discussed.

We report an angle-resolved photoemission study of a series of hole- and electron-doped iron-based superconductors, their parent compound $\mathrm{Ba}{\mathrm{Fe}}_2{\mathrm{As}}_2$, and their cousins $\mathrm{Ba}{\mathrm{Cr}}_2{\mathrm{As}}_2$ and $\mathrm{Ba}{\mathrm{Co}}_2{\mathrm{As}}_2$. We focus on the inner hole pocket, which is the hot spot in these compounds. More specifically, we determine the energy ($E$)-dependent scattering rate $\mathrm{\Gamma }\left(E\right)$ as a function of the $3d$ count. Moreover, for the compounds ${\mathrm{K}}_{0.4}{\mathrm{Ba}}_{0.6}{\mathrm{Fe}}_2{\mathrm{As}}_2$ and $\mathrm{Ba}{\mathrm{Cr}}_2{\mathrm{As}}_2$, we derive the energy dependence of the renormalization function $Z\left(E\right)$ and the imaginary part of the self-energy function $\mathrm{Im}\mathrm{\Sigma }\left(E\right)$. We obtain a non-Fermi liquidlike linear in energy scattering rate $\mathrm{\Gamma }(E\gg k_{B}T)$, independent of the dopant concentration. The main result is that the slope $\beta =\mathrm{\Gamma }(E\gg k_{B}T)/E$ reaches its maxima near optimal doping and scales with the superconducting transition temperature. This supports the spin fluctuation model for superconductivity for these materials. In the optimally hole-doped compound, the slope of the scattering rate of the inner hole pocket is about three times bigger than the Planckian limit $\mathrm{\Gamma }\left(E\right)/E\approx 1$. This result, together with the energy dependence of the renormalization function $Z\left(E\right)$, signals very incoherent charge carriers in the normal state which transform at low temperatures to a coherent unconventional superconducting state.

The two-dimensional transition-metal dichalcogenide $1T\text{−}{\mathrm{TaS}}_2$ is a complex material standing out for its puzzling low temperature phase marked by signatures amenable to both Mott-insulating and charge-density wave states. Electronic Mott states, coupled to a lattice, respond to coherent optical excitations via a modulation of the lower (valence) Hubbard band. Such dynamics is driven by strong electron-phonon coupling and typically lasts for tens of picoseconds, mimicking coherent structural distortions. Instead, the response occurring at the much faster timescale, mainly dominated by electronic many-body effects, is still a matter of intense research. By performing time- and angle-resolved photoemission spectroscopy, we investigated the photoinduced phase of $1T\text{−}{\mathrm{TaS}}_2$ and found out that its lower Hubbard band promptly reacts to coherent optical excitations by shifting its binding energy towards a slightly larger value. This process lasts for a time comparable to the optical pump pulse length, mirroring a transient change of the onsite Coulomb repulsion energy ($U$). Such an observation suggests that the correction to the bare value of $U$, ascribed to the phonon-mediated screening which slightly opposes the Hubbard repulsion, is lost within an interval of a few tens of femtoseconds and can be understood as a fingerprint of electronic states largely decoupled from the lattice. Additionally, these results enforce the hypothesis, envisaged in the current literature, that the transient photoinduced states belong to a sort of crossover phase instead of an equilibrium metallic one.

We report on an infrared magnetospectroscopy study of ${\mathrm{Pb}}_{1-x}{\mathrm{Sn}}_x\mathrm{Se}$, a topological crystalline insulator. We have examined a set of samples, all in the inverted regime of electronic bands, with the tin composition varying from $x=0.2$ to 0.33. Our analysis shows that the observed response, composed of a series of interband inter-Landau-level excitations, can be interpreted and modeled using the relativistic-like Hamiltonian for three-dimensional massive Dirac electrons, expanded to include diagonal quadratic terms that impose band inversion. In our data, we have not found any clear signature of massless electron states that are present on the surface of ${\mathrm{Pb}}_{1-x}{\mathrm{Sn}}_x\mathrm{Se}$ crystals in the inverted regime. Reasons for this unexpected result are discussed.

We study the different ways of introducing light-matter interaction in first-principle tight-binding (TB) models. The standard way of describing optical properties is the velocity gauge, defined by linear coupling to the vector potential. In finite systems a transformation to represent the electromagnetic radiation by the electric field instead is possible, albeit subtleties arise in periodic systems. The resulting dipole gauge is a multi-orbital generalization of the Peierls substitution. In this work we investigate the accuracy for both pathways, with particular emphasis on gauge invariance, for TB models constructed from maximally localized Wannier functions. This approach accurately captures the light-matter interaction close to the Fermi level. Focusing on paradigmatic two-dimensional materials, we construct first-principle models and calculate the response to electromagnetic fields in linear response and for strong excitations. Benchmarks against fully converged first-principle calculations allow for ascertaining the accuracy of the TB models. We find that the dipole gauge provides a more accurate description than the velocity gauge in all cases. The main deficiency of the velocity gauge is an imperfect cancellation of paramagnetic and diamagnetic current. Formulating a corresponding sum rule however provides a way to explicitly enforce this cancellation. This procedure corrects the TB models in the velocity gauge, yielding excellent agreement with dipole gauge and thus restoring gauge invariance.

We study the edge states of twisted bilayer graphene and their topological origin. We show that the twisted bilayer graphene has special edge states associated with the moiré pattern, and the emergence of these moiré edge states is linked with the sliding Chern number, which describes topological charge pumping caused by relative interlayer sliding. When one layer of the twisted bilayer is relatively slid with respect to the other layer, the edge states are transferred from a single band to another across the band gap, and the number of edge states pumped in a sliding cycle is shown to be equal to the sliding Chern number of the band gap. The relationship can be viewed as a manifestation of the bulk-edge correspondence inherent in moiré bilayer systems.

Based on the first-principles simulations within the framework of density functional theory, we predict that ${\mathrm{CoN}}_4{\mathrm{C}}_2$ is a single-atom-thick two-dimensional material, which is composed of ${\mathrm{CoN}}_4$ unit and ${\mathrm{C}}_2$ dimer. The cohesive energy and phonon dispersion calculations of ${\mathrm{CoN}}_4{\mathrm{C}}_2$ monolayer, as well as molecular dynamics simulation, have been carried out and its stability is confirmed. The electronic states near the Fermi energy are dominated by Co $3d$ orbital states and the $2p$ orbitals of C and N atoms make a small contribution. The unusual electronic feature of ${\mathrm{CoN}}_4{\mathrm{C}}_2$ is the flat band state close to the Fermi energy, which is derived from Co ${\mathrm{d}}_{z^2}$ partial orbital. With the tensile strain increasing, a phase transition from nonmagnetic state to magnetic state is induced. At the biaxial tensile strain of 8.6%, the flat band is pushed to the Fermi energy. Moreover, the effect of electron correlation correction on the electronic structure of ${\mathrm{CoN}}_4{\mathrm{C}}_2$ is also discussed by the SCAN meta-GGA method and $\mathrm{GGA}+U$ method. Our results demonstrate that tensile strain is another knob to realize the flat band state at the Fermi level, similar to the twist angle in twisted bilayer graphene reported recently.

We investigate the effect of local geometry and environment on the electronic spin of nitric oxide (NO) on Au(110)-($1\times 2$) using scanning tunneling microscopy/spectroscopy, density functional theory, and numerical renormalization group calculations. The molecules adsorb on the bridge or on-top sites, and the Kondo resonance is observed only for the latter. This indicates that the local geometry influences the spin state of NO on the surface. The Kondo resonance is accompanied by enhanced satellite peaks due to the interplay with vibrational excitation. In addition, we find that the Kondo resonance appears as a peak or a dip, depending on the local environment of the molecule. The calculations reveal that the NO/Au(110) system is represented by a two-orbital Anderson model, where the $2{\pi }^{*}$ orbitals compete for the unpaired electron to host the Kondo resonance. We propose that a subtle interaction between the NO molecules may play a critical role in determining the host of Kondo resonance and, consequently, the spectral shape.

Rhombohedral $N=3$ trilayer graphene on hexagonal boron nitride hosts gate-tunable, valley-contrasting, nearly flat topological bands that can trigger spontaneous quantum Hall phases under appropriate conditions of the valley and spin polarization. Recent experiments have shown signatures of $C$ = 2 valley Chern bands at 1/4 hole filling, in contrast to the predicted value of $C$ = 3. We discuss the low-energy model for rhombohedral N-layer graphene ($N$ = 1, 2, 3) aligned with hexagonal boron nitride subject to off-diagonal moiré vector potential terms that can alter the valley Chern numbers. Our analysis suggests that topological phase transitions of the flatbands can be triggered by pseudomagnetic vector field potentials associated to moiré strain patterns, and that a nematic order with broken rotational symmetry can lead to valley Chern numbers that are in agreement with recent Hall conductivity observations.

Twisted bilayer graphene (TBG) has been experimentally observed to exhibit almost flat bands when the twisting occurs at certain magic angles. In this Letter we show that in the approximation of vanishing AA coupling, the magic angles (at which there exist entirely flat bands) are given as the eigenvalues of a non-Hermitian operator, and that all bands start squeezing exponentially fast as the angle $\theta$ tends to 0. In particular, as the interaction potential changes, the dynamics of magic angles involves the nonphysical complex eigenvalues. Using our new spectral characterization, we show that the equidistant scaling of inverse magic angles is special for the choice of tunneling potentials in the continuum model, and is not protected by symmetries. While we also show that the protection of zero-energy states holds in the continuum model as long as particle-hole symmetry is preserved, we observe that the existence of flat bands and the exponential squeezing are special properties of the chiral model.